Subject: Chemistry 

  Chemistry Demonstrations    Required time:   Disappearing Smoke: 20 minutes  Bouncy Ball: 30 to 40 minutes 

  Goals  ● ●

Demonstrate the difference between a physical and chemical reaction to students  Introduce students to chemical reactions and limiting reagents via the borax bouncy ball  activity 

 

Materials Needed  Demonstration 1: Disappearing Smoke  ● Giant jar  ● Matches  ● Incense (or something to burn and create smoke)  ● Isopropyl Alcohol  ● Safety glasses and gloves    Demonstration 2: Bouncy Ball   ● Borax   ● White glue (Elmer's Glue) or Blue or Clear school glue (makes a translucent ball)  ● Warm water  ● 2 small plastic cups or other containers for mixing  ● Spoon to stir the mixture  ● Measuring spoons  ● Zip-lock plastic baggie  ● Cornstarch (optional)  ● Food coloring (optional)   

  Introduction (~10 to 15 minutes)  Explain to students that:  ● Chemistry is a science of trial and observation:   ○ Scientists mix things together in different amounts under different conditions,  and see what happens. Then they try to explain what they see using what they  know about matter -- including atoms, the smallest part of life.   ● Physical and chemical change is different 

Chemical change is moving around atoms.   ■ Comparable to making something with individual ​lego​ pieces. We can  break lego pieces apart and put them back together differently. Atoms are  essentially like this -- they can make and break connections at the  smallest level  ○ Physical change DOES NOT involve moving around individual atoms  ■ Changes can be undone, like water -> ice  ■ Comparable to having large pieces made from legos and these large  pieces being reshuffled around (individual legos are not moving around)  It may be tricky to differentiate between a chemical and a physical change  ○ Key question to ask is, "Can I change this back to what I had before without  putting in too much energy, or without having to go through a lot of trouble?” ---->  Physical changes don’t form new products!  ○

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Disappearing Smoke Demo  PLEASE USE GLOVES AND SAFETY GOGGLES WHEN DOING THIS DEMO!!    Outline:   1. Prepare the jar of smoke ​before​ class.  2. Start of class, demo with another mentor the burning smoke. Then discuss.    Procedure [source 1]  1) Light the tip of incense and place in giant jar. Cover opening of the jar enough to let smoke  fill up the jar. It will take from thirty seconds to a minute to completely fill up the jar.   2) Once the jar is filled, pour some isopropyl alcohol into the jar and shake the jar to induce  vaporization of the alcohol. This will allow the alcohol to vaporize. Allow the vapor to fill as  much of the jar as possible.  3) Light a match, and carefully drop into the jar, and watch as the smoke disappears.      Alternative procedure without incense  1) Pour isopropyl alcohol into jar, ~2 capfuls. Close lid of jar and shake for 2-3 minutes. Make  sure to shake, such that the alcohol vaporizes and spreads throughout the container.   2) Once ready, unscrew the cap of the jar (or loosen the jar cover if there is no cap), light a  match, and drop it into the jar. (This works best with two people, with one person working with  the match, and the other opening the top of the jar once the match is lit, to prevent any of the  vapor from escaping.) If this works successfully, you should be able to witness a "rising flame  demo". If you don't get to see this, don't fret because you can still continue on with the demo.    3) If the match starts to smoke upon making contact with the alcohol at the bottom of the jar,  make sure to quickly fasten the lid onto the jar, and let the smoke collect in the jar. It may take a 

few minutes for enough smoke to visibly fill up the jar. (If the first match didn't smoke enough,  you can try shaking the jar again, and dropping in a second one. Jar should start filling up with  smoke.)   4) Make sure you continue shaking even while the smoke is collecting, as this will allow the  alcohol to vaporize (and remain vaporized) throughout the jar.  5) Once enough smoke has collected in the jar, you do as you did before. Light a match and  drop it in, and you should see the smoke disappear from the jar.      Discussion​:  Combustion is an example of a chemical reaction (resulting in a chemical change). However,  the isopropyl alcohol vaporizing is an example of a physical change. The alcohol had to vaporize  (the way water vapor forms when water boils--kind of like the steam) and we helped speed up  this process by shaking the jar.    What characteristics did the demo have of a chemical reaction did it have? The smoke once the  alcohol combusted.      Concept check question: So to review, what type of reactions did this demo exhibit? Are there  any questions?   

Make a Bouncy Ball Demo   You may want to use gloves for this experiment, as it can get a bit sticky.    Outline:  Have materials laid out on tables already. We’ll need spoons, a container of glue, some borax in  a bowl. There will be s ​ even​ bowls per two people: three for glue (to run three trials) and one for  holding the borax solutions in each of the trials. Mentors are r​ ecommended​ to join a group and  do the trials alongside the students; they could even run the trial with the cornstarch so students  will be able to see what happens with the addition of cornstarch. Goal is to run ​two​ trials, a third  if people finish and want to do a third try.      Procedure​ ​[source 1 + feedback from trials]  1) One cup will contain the borax mixture and the other a ball mixture  2) Pour 2 ​ tablespoons (or 6 teaspoons) of warm water​ and ½ teaspoon b ​ orax​ p ​ owder​ into the  cup for the borax solution. Stir the mixture to dissolve the borax.    3) Pour 1 ​ tablespoon (or 3 teaspoons​)​ of glue​ into the cup for the ball mixture. You may add  food coloring to give the bouncy ball color. Add ½ ​ teaspoon of the borax​ ​solution​ you just  made. ​Do not stir.​ Allow the ingredients to interact on their own for ​20-30 seconds​ and then stir  them together to fully mix. Once the mixture becomes impossible to stir, take it out of the cup  and start molding the ball with your hands.  4) The ball will start out sticky and messy, but will solidify as you knead it.   4.5) If it's still sticky, add just a little bit more borax solution to the surface, and continue 

molding it. Repeat this step as many times as necessary, but be careful, as adding too much  borax will be detrimental to the project.  5) Once the ball is less sticky, it is ready to be bounced!    6) You can store the ball in the ziplock baggie for later use.    Alternate Procedure (to work in pairs)​ [source 2]  Proportions for this mixture:  ● 12 teaspoons​ of warm water  ● 6 teaspoons​ of glue  ● 1 teaspoon​ of borax  When you add the borax solution to the glue, you will have to add 1 ​ teaspoon​ of the solution    If finish early, going over and beyond:  Try making another bouncy ball, but this time include cornstarch (1 tablespoon or 3 teaspoons  is the recommended amount). Or, you can try it again but with different proportions of the  ingredients. What happens?    Suggestions:  As students are working, try to explain what is happening in the reaction. Group leader will go  over this information when group recollects.   "​Glue is a polymer called polyvinyl acetate. Polymers are long chain-like molecules made up of repeating  units. The original glue chains were sticky and runny. Adding the borax, a crosslinker, tied some of the  polymer chains together, restricting their flow and creating the putty like material students worked with.  The crosslinking of the polymer is an example of a chemical change. The polymer chains are bonded  together changing the properties of the polymer to make it more viscous (flows more slowly) and more  bouncy." [source 3] 

Image source: http://www.emeraldinsight.com/ 

  Discussion:  1. Was this a chemical reaction or a physical reaction?  ● Chemical reaction because of polymer cross linking. A little tricky, but in order to form  these cross links, there are a few bonds that have to be broken first, and since bonds are  broken it must be a chemical reaction.     

  2. Give an example each of a chemical reaction and of a physical reaction in everyday life.  Explain why they are an example of that type of reaction.    

References:  Disappearing Smoke  http://www.youtube.com/watch?v=3BaFpyE8sMo​ [1] 

Bouncy Ball  http://chemistry.about.com/od/demonstrationsexperiments/ss/bounceball.htm​ [1]  http://www.curiosityzone.com/Experiments/Experiments/bouncy%20%20balls.html​ [2]  http://agpa.uakron.edu/p16/lesson.php?id=betterbouncer&pg=procedures​ [3]                          [Bonus demo if there is time]  C) Pepsi Mentos (physical reaction) - will have to be done outdoors   (10 minutes)  Materials:  ● Liter bottle of Diet Coke  ● Mentos    1) Place Diet Coke in an open area   2) Drop mentos into Diet Pepsi, and watch it fountain out  Run demo about 5 times:  ● Do it once with 2-3 mentos. Discuss. Type of reaction? Physical. Why? Was anything  produced? Was anything consumed? Nucleation site. Kind of like pushing the first  domino in a stack of dominoes. Without that first one none of them will fall.  ● Do three more times. Let students pick a number and predict what will happen. Demo.  ● Fifth bottle, predict what will happen, then drop mentos in bottle and close cap.      Discussion:  Does anyone know why this is a physical reaction?  

The mentos does not cause any bonds to be made or broken. What happens is that the carbon  dioxide in the soda congregates around the mentos, and then as the gas is released, the liquid  from the bottle rushes up.